This invention relates to voltage regulating apparatus for accurately controlling the root-means-square (RMS) voltage applied to a load. More particularly, this invention relates to improvements in voltage regulating circuitry whereby the RMS output voltage can be maintained substantially constant even during rapid changes in the AC line voltage.
For many applications, it is desirable to maintain the RMS voltage applied to a load as constant as possible notwithstanding wide and rapid variations in the AC power line voltage. In the field of photographic printing, for instance, the voltage applied to the filament of the printing lamp is an extremely critical parameter in the printing operation, for even relatively small changes in the applied voltage applied to the lamp can produce very noticeable results in print quality. Particularly, when the exposure times are short, it is important that any voltage regulating apparatus used to control the lamp voltage be capable of compensating for rapidly occurring variations in line voltage (e.g. occurring within one-half cycle). Such rapid variations typically result from short-lived power surges and transients occurring on the power line voltage.
Generally speaking, voltage regulator devices heretofore have been incapable of responding fast enough to compensate for substantial variations in line voltage occurring within a half-cycle of the line voltage frequency. Some of the faster regulators depend upon the generation of DC signals representative of the average value or the peak value of either the AC line voltage or the load voltage or various combinations of each. Although regulators dependent on the generation of DC signals display somewhat faster response than most regulators, they still have a serious limitation in their ability to respond rapidly to changes in the AC power line voltage. This is because the rectifying and filtering networks required to produce the DC signals cannot respond instantaneously to rapid changes in the AC power line voltage. Attempts to reduce time constants in these networks and thereby improve the speed of response often result in a degradation of the linearity of the regulator. Thus regulators of this type are often characterized by poor transient response and good linearity or vice versa.
In addition to the relatively slow response which is characteristic of conventional voltage regulators, many of these use unipolarity signals to trigger a phase controlled switch (e.g. a triac) which serves to synchronously switch the voltage applied to the load. Such regulators require relatively large currents for reliable triggering.